GS-1 Question 5

[alex.bell7015]

I'm having a little trouble understanding the explanation given for Question five in the Physical Sciences portion of GS-1.

The question asks where in a pipe the pressure is the highest and the answer is given as the region where the cross sectional area is the greatest

However, considering the scenario rationally, if the flow is moving from left to right, the pressure MUST be highest at the leftmost end of the pipe. All fluids flow from highest pressure to lowest, regardless of pipe diameter, flow regime or velocity profile.

If section D where to be the section of highest pressure, the flow arrows should be reversed on the diagram.

[admin]

There is no question that within a pipe there are pressure changes that occur according to the equations of continuity and Bernouilli's. You are referring to something different which was not discussed in this problem. If you have a pump at one end of a pipe system and the other end of the pipe system is open, then you have a high pressure to low pressure (classical) system. However, within the pipe system, there can be fluctuations in pressure (in each case, these fluctuations in pressure can still be higher that the pressure, let's say, at the far right end).

Thus you have a garden hose. The pressure at the pump is higher than the pressure at the outlet. Now if you step on some part of the hose, partly occluding the lumen, then clearly pressure rises in the segment of the hose between the pump and your foot (the pressure rises to try to push through the resistance that you created). As the water goes through the smaller diameter, the velocity is greater and the water applies less pressure against the walls of the hose as it quickly rushes through to the other side, etc.

You are not the only one to have that question, see the following link:

www.newton.dep.anl.gov/newton/askasci/1993/eng/ENG23.HTM


NB In future, please post questions about GS exams in the section that states the name of the exam: for example GS-1 then Physical Sciences. That way you can see the questions that others have asked and it will be easy for students to find your question. Thanks for understanding.